1. Field of the Invention
This invention relates to electronic components, and particularly to cooling devices for microprocessors.
2. Description of Background
A typical air cooling scheme for high power microprocessors includes a heat sink in close contact with a microprocessor chip to be cooled. The heat sink conducts heat from the chip to heat sink fins where the heat is transferred to the atmosphere by convection. A thermal interface material (TIM) is used to achieve optimal thermal contact between the chip and the heat sink. The TIM has a relatively poor thermal conductivity (about 4 W/mK), however, so it is desirable to minimize its thickness to optimize the thermal contact between the chip and the heat sink. TIM material is typically made of conducting metal oxide particles of a given size (50 um) embedded in an oil-like medium. New generation of TIM materials behave more like a soft solid rather than high viscosity paste.
To be capable of dissipating the large amount of heat generated by the chips, the heat sink fins are made larger, and are often significantly larger than the chip itself. In such a case, the heat sink utilizes a heat spreader, typically made from a highly conductive metal such as copper. The heat sink having large fins and a heat spreader is often too heavy to be supported directly by the chip it is cooling, so the heat sink is attached to, and at least partially supported by, the circuit board to which the chip is attached. To accommodate different expansion rates between the chip and heat sink during power on/power off cycles, the gap between heat sink and chip must be controlled with elaborate mechanical system design. In present designs, in order to maintain minimum gap the heat sink is pressed against the particles contained in a TIM material with a preload of about 50 to 100 N. Particles of size 50 um help to maintain a minimum gap of about 50 um. The preload is generally not desirable and can be detrimental to the electronic components such as solder balls that support the chip. More importantly, cyclic changes in the mechanical clearance between the chip and the heat spreader can result in a pumping action of TIM that can lead to voids being formed in the TIM and eventually to chip failure. In the case of TIM materials that are not paste-like but have a viscoplastic characteristics the cyclic changes in mechanical clearance can lead to fatigue failure of a thermal interface.
What is needed is a heat sink with a heat spreader that is capable of dissipating the necessary amount of heat, and which includes a minimal thickness of TIM at the heat sink interface to the chip.